High reliability motor system

ABSTRACT

A brushless electric motor system comprises a rotor and a stator comprising poles. Electrical phase windings have coils (U 1 - 6 , VI- 6 , W 1 - 6 ) wound around the poles. Power switches (T 1 -T 24 ) controlled by a control device supply electric current to the windings from positive and negative rails connected to power supply. For each phase at least one group of four power switches arranged in a H-bridge configuration is provided. The coils of each phase winding are preferably divided into winding groups (U 1 - 3 , Ua- 6 , VI- 3 , V 4 - 6 , W 1 - 3 , W 4 - 6 ) and the electric conductor of each winding group is then electrically insulated from the conductor of the other winding groups. Then four power switches arranged in an H-configuration is provided for each winding group. The use of power switches in H-bridge configurations allows the faulty windings or winding groups to be disabled and that the rest of the windings or coils can be used for driving the rotor, this giving the motor system a high reliability. The current supplied to other windings or winding groups can then be increased to compensate for the faulty group. The coil groups can be separated from other coil groups by unwound stator poles. Current sensors ( 303 ) can sense the current in each winding group and be used to detect whether the currents are too high. The sensed currents can be used to identify fault conditions in the system so that then suitable switches can be disabled, disconnecting a faulty winding group.

TECHNICAL FIELD

[0001] The present invention generally relates to brushless electricmotor systems having high reliability and to methods of designing suchmotor systems.

BACKGROUND OF THE INVENTION

[0002] Brushless electric motor systems have several failure modes, someof which are illustrated in FIG. 1. The illustrated motor has threestator phases 101, 102 and 103. Each phase is illustrated as a coilhaving only two turns such as 104 and 105. The phase coils pass throughslots in a laminated iron stator pack 106. One end of each of the phasecoils is connected to a common node at 107. The other ends 108, 109 and110 of the phase coils are connected to a three phase bridge having sixpower switches 111, . . . , 116.

[0003] The fast switching of the transistors give high voltage ringingtransients in the phase coils. This creates high local voltagesspecially where the phase windings cross each other on the way from oneslot to another, and may create short circuit paths as illustrated at117. The insulation against ground can also break as illustrated at 118.The insulation within a coil can fail as indicated at 119, creating ashort circuit loop illustrated as a thicker line at 105. The transistorscan fail to operate and then are in a permanent closed or openconfiguration.

[0004] All these failure modes will completely disable the operation ofa conventional motor system.

SUMMARY OF THE INVENTION

[0005] It is an object of the invention to provide a brushless electricmotor system having dramatically reduced risk of phase to phaseshortages.

[0006] It is another object of the invention to provide a brushlesselectric motor system that is capable of delivering substantial torqueor power even with a ground to phase shortage.

[0007] It is another object of the invention to provide a brushlesselectric motor system that is capable of delivering substantial torqueor power even with a short circuit inside a phase winding.

[0008] It is another object of the invention to provide a brushlesselectric motor system that is capable of delivering substantial torqueor power even with a permanently open power switch.

[0009] It is another object of the invention to provide a brushlesselectric motor system that is capable of delivering substantial torqueor power even with a permanently shortened power switch.

[0010] It is another object of the invention to provide a brushlesselectric motor system that is capable of delivering almost full powerover a long time even when various types of failure causes abnormalheating of winding parts.

[0011] The above objects are achieved by the invention, thecharacteristics of which appear from the appended claims.

[0012] The brushless electric motor system considered herein, that canbe a rotational or linear motor, generally comprises a rotor or linearslide having north and south poles and a stator comprising poles, alsocalled stator teeth, and electrical phase windings. Each phase windingincludes a plurality of coils, each coil wound around an individual oneof the stator poles. A switching network controlled by a controller orcontrol device performs the electronic commutation. It comprises powerswitches, each power switch having one switch terminal connected to apole or terminal of an electric power source and another switch terminalconnected to an end of one of the coils. Each of the power switches thuseither is closed, i.e. connects, as controlled by a control signal fromthe control device, the end of the associated coil to the respectivepole or terminal of the electric power source, or is open, disconnectingthe coil from the electric power source. The control device controls theswitching network, in particular the power switches thereof, to supplythe phase windings with electric current or power at times selected toachieve a torque or force on and thereby a movement of the rotor orlinear slide. The coils of each phase winding are electrically insulatedfrom coils in the other phases.

[0013] The switching network can for each phase comprise at least onegroup of four power switches that are arranged in an H-configuration andare connected between a positive conductor or rail connected to apositive pole of the power supply and a negative conductor or railconnected to a negative terminal of the power supply. The coils of eachphase winding are divided into winding groups and then each windinggroup is electrically insulated from the other winding groups. Theswitching network can then for each winding group comprise four powerswitches arranged in an H-configuration between the positive conductoror rail and the negative conductor or rail. An individual control of thepower supplied to the phase windings or coil groups hat is allowed byhaving the power switches and windings or coil groups connected to eachother in this type of bridges, this furthermore allowing that windingsor coil groups can be individually disconnected from the power supply inthe case where there is some fault or failure of some componentassociated with the windings or coil groups. This allows that electriccurrent can still be supplied to other windings or coil groups,

[0014] Each coil of each one of the groups can be separated from thecoils in the other group or groups by unwound stator poles to give anadditional spatial electrical insulation between the groups. Each coilof each one of the groups can be connected to other coils in the samegroup and/or to the switching network through high current limitingdevices that preferably are fuses. These features obviously enhance thereliability of the electric motor.

[0015] The control of currents in the windings or coil groups can usevalues obtained from current sensors sensing the current in each phasewinding or in each winding group, in particular sensing whether theabsolute values of the currents are too high.

[0016] The switching network and in particular the control device canthus be arranged to detect whether the magnitudes of the sensed switchedcurrents exceed a safety level. The control device can then, byanalyzing the sensed currents, identify various fault conditions thatinclude shorts inside phase windings, shorts from phase windings toground and failures of the power switches. It can then disable those ofthe switches that drive faulty phases or winding groups and use thestill operating parts of the brushless motor system to ensure that thebrushless motor system can operate to move the rotor or linear slide inspite of fault conditions albeit with a reduced peak performance.

[0017] The control device can then act according to a control method inwhich the performance of the motor system is optimized for faultconditions identified in system components. It can then use informationfrom the temperature sensors to adjust, in particular increase, stillcontrollable currents flowing through phase windings and/or coil groupsso that a maximum output power/torque/force or a maximum performance inregard of the torque or force is obtained without surpassingpredetermined temperature limits. The predetermined temperature limitsused can be based on a relatively negligible risk for additional faultsor failures of system components during an expected maximum time duringwhich the brushless electric motor system must be able to operate afteridentifying one fault condition. These considerations involving anmaximum time and predetermined temperature limits can be used in amethod of designing an electric motor system exhibiting a highreliability.

[0018] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the methods, processes, instrumentalities and combinationsparticularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] While the novel features of the invention are set forth withparticularly in the appended claims, a complete understanding of theinvention, both as to organization and content, and of the above andother features thereof may be gained from and the invention will bebetter appreciated from a consideration of the following detaileddescription of non-limiting embodiments presented hereinbelow withreference to the accompanying drawings, in which:

[0020]FIG. 1 is a schematic illustrating the configuration of electricwindings of a conventional brushless electric motor system.

[0021]FIG. 2 is a sectional view of a brushless electric motor havinghigh reliability,

[0022]FIG. 3 is circuit diagram of the power electronics and phases ofthe motor of FIG. 2 in which various faults are indicated,

[0023]FIG. 4 is a detailed circuit diagram corresponding to half thecircuit diagram of FIG. 3 and also showing detectors and a controller,

[0024]FIG. 5 is a circuit diagram similar to that of FIG. 3 but alsohaving additional switches for protection/low speeds,

[0025]FIG. 6 is a diagram showing graphs of controlled currents andresulting torques in a motor system in which one winding group isdisconnected, for a control method giving a maintained high total outputtorque,

[0026]FIG. 7 is a diagram similar to that of FIG. 6 showing graphs for adifferent control method,

[0027]FIG. 8 is a diagram showing graphs of the emf E, the brake torqueT and the current I in a current loop caused by a permanently shortedswitch,

[0028]FIG. 9 is a diagram similar to that of FIG. 8 for a case where agroup of three coils have been shorted,

[0029]FIG. 10 is a sectional view of a brushless electric motor havinghigh reliability according to a second embodiment,

[0030]FIG. 11 is a sectional view of a brushless electric motor havinghigh copper fill factor, and

[0031]FIG. 12 is a sectional view of a brushless electric motor havingincreased coil ventilation channels.

DETAILED DESCRIPTION

[0032] In FIG. 1 the principle layout of the electric windings and theirconnection in a conventional brushless electric motor is illustrated.The illustrated motor has three stator windings 101, 102 and 103connected to three phases and arranged in slots between poles of alaminated iron stator pack 106, the poles projecting towards therotation axis, not shown, of the motor. Each phase winding isillustrated as a coil having only two turns such as 104 and 105. All thephase coils have one end interconnected, at a common node 107. Any partof any winding is connected to all other parts of all windings throughmetallic conductors. The other ends 108, 109 and 110 of the phase coilsare connected to a switching circuit, in particular to theinterconnection nodes of pairs of power switches 111 and 112, 113 and114, 115 and 116, the switches of each pair connected in series betweentwo rails connected to a power supply, not shown.

[0033] In FIG. 2 a cross-section of a brushless electric motor systemhaving a high reliability is shown. Unlike the motor of FIG. 1, thestator windings are divided into groups. The phase denoted by U has sixcoils U1-U6 divided into two groups U1-U3 and U4-U6, each groupcomprising three coils. The electric conductors of the coils of eachphase winding group are connected in series with each other. Theidentifications of the coils are in the figure indicated on thelaminated iron poles like 201 around which they are wound. Coil W4 isfor example shown as sections 211 and 212. Then also the stator polesare divided in corresponding pole groups, each of the stator pole groupscarrying the coils of an individual one of the phase winding groups.Each of the resulting six stator pole groups is insulated from the othergroups in the sense that there is no metallic conductive connectionbetween them. Each of these six stator pole groups, for instance thepole group carrying the winding group U4-U6, is separated from theadjacent groups, such as the pole groups carrying the coils V4-V6 andW1-W3, by two unwound stator poles such as 202 and 203 located at eitherside of the pole group. The unwound stator poles can as illustrated inthe figure be narrower than the wound stator poles and thereby extendover a smaller angle at the circumference of the rotor. 20 rotor magnetslike 204 and 205 are arranged at the circumference of the rotor.

[0034] The mechanical assembly shown in FIG. 2 permits a compact packagewith a reduced risk for cable and connection faults, as the connectionbetween the motor and the power electronics system only includeselectric lines for the supply voltage and for control signals. Anarrangement for forced cooling is indicated as an input 207 of coolingair or fluid and channels such as 208 for the medium on its way to theoutlet 209.

[0035] Alternatively, the motor can be cooled by ventilation air movingaxially between the coils and around the magnets.

[0036] The circuit diagram of FIG. 3 shows the power switches and theirconnection to the windings of the motor shown in FIG. 2. The powerswitches comprise 4 modules each having 6 power switches or transistorsT1, T2, . . . , T24, which often is advantageous as the large productionvolumes of 6-transistor modules make them economical. The 24 transistorsare connected to act as 6H-bridges, each H-bridge including 4transistors and driving one phase winding group. For example, the phasewinding group W4-6 is driven by transistors T1-T4. One end of theelectric conductor in each phase winding group, e.g. W4-6, is connectedto the interconnection node of a first pair of the power switches suchas T1 and T2 and the opposite end is connected to the interconnectionnode of a second pair of the power switches such as T3 and T4. Theswitches of each pair is connected in series between two rails connectedto a power supply, not shown. The current through the windings of eachpole group is checked by a current sensor such 303 for the phase windinggroup W4-6. Controlling a current and the intensity thereof flowingthrough a coil or an inductance such as the conductor of winding groupusing a 4 switch H-bridge and a current sensor is well known in the art.

[0037]FIG. 4 is a more detailed circuit diagram of an improved versionof the power and signalling connections of FIG. 3. Only half the systemis shown. Slow fuses like 401 are inserted in series with the threecoils in each phase winding group, e.g. as illustrated one slow fusebetween each pair of adjacent coils, i.e. two fuses per phase windinggroup.

[0038] The power switches are encapsulated in hex switch assemblies 406and 407, each switch assembly thus including six switches. A motorcontroller 403 controls the six switches in each assembly throughcontrol lines like 404. Many commercially available power switchassemblies contain safety sensors, not shown, that will disable one orall of the switches if the current flowing through anyone of theswitches of the assembly becomes greater than a safety level. At thesame time an alarm signal will be issued, for instance on a line 406, tothe controller 403.

[0039] The controller 403 must have information on the angle position ofthe rotor in relation to the stator of the motor through some kind ofposition sensor such as Hall effect elements, an encoder or a resolver.This information is provided through lines 405. Information on thecurrent intensities in the different phase winding groups are suppliedthrough current sensors like 303, and information on the temperature ineach phase winding are supplied by sensors like 402 to the controller403. The sensors should preferably permit a rather high resolution, atleast four bits, to permit a measurement of the coil temperature up totemperatures close to short term failure temperature of the windinginsulation. Motor protection sensors often have almost binarycharacteristics, such as thermistors with a low resistance below 140° C.and a dramatically higher resistance above 150° C. Such sensors are notsuitable for the more complex task of monitoring temperatures in motorsthat deliver power during serious fault conditions.

[0040] Some of the error modes of a brushless electric motor system havebeen indicated in FIG. 3. Some examples of the magnitude of lostperformance, coil heating and failing torque will be given for aconvection cooled motor of some 2 kW. Other motors sizes and othercooling systems will give different results.

[0041] A purpose of the invention is to provide a brushless electricmotor system that is capable of delivering substantial output torque oroutput power even for a permanently open power switch. If a power switchsuch as T13 will be kept permanently passive by a fault shown as 304,all of the four switches T13-T16 should be disabled. In someapplications, the functional switch pair T14-T15 could be used toprovide some torque. This case is not discussed in the followingdescription. Disabling T13-T16 will eliminate the current in the phasecoils W4-W6. If the phase currents in the remaining pole groups are keptat normal values, the torque will vary between 100% and 66% of thetorque from a normally operating motor.

[0042] In convection cooled motors, the thermal resistance between motorframe and the ambient is an important part of the thermal resistancebetween winding and ambient. In such cases the currents in the operatingparts of the faulty motor can be increased almost to the level where thetotal copper loss from the faulty motor can be kept at the same level asfor a correctly operating motor. One example of this is illustrated bythe plots in the diagram of FIG. 6. In this example, one of the two Uwinding groups is disabled. The top graphs show the phase currents likeI_(u) _(—) _(normal) in a normally operating motor. In the case shownbelow the top graphs, the current like I_(u) _(—) _(adjusted) in theremaining operating pole groups have been increased by a factor commonfor all phase winding groups that varies with the electric angle in sucha way that the total copper loss in the active phases are identical tothe total loss for a normally operating motor. This correction willcause the torque T_(tot) _(—) _(adjusted) to vary between 100% and 81.6%of that of a correctly operating motor as seen in the lower-most graph.The torques like T_(w) _(—) _(adjusted) from each phase are shown by thegraphs in the middle of the diagram. The graphs of FIG. 6 illustrate theprinciple only. The optimal adjustment of the phase currents will dependon thermal and other characteristics of the motor system.

[0043] The adjustment of the phase current intensities is made by havingthe controller 403 control the power switches accordingly.

[0044] The strategy illustrated by the graphs of FIG. 6 comprising thatthe current intensities are adjusted to produce the same copper loss forall rotor angles is relevant for systems which partly operate at zero orvery low speeds, for example for setting the angle of a control orguiding surface of an aircraft. The diagram of FIG. 7 illustrates aphase current adjustment for motors that always rotates at speeds forwhich the phase current frequency is so high that the thermal inertia ofthe stator coils dampens out the angle dependent variations of theheating. In such cases, the phase current adjustment can be made in sucha way that the total copper losses over one turn is the same as that ofa properly operating motor system. Assuming that a constant no-rippletorque is desired, that the thermal resistance from the motor to theambient totally dominates over the thermal resistance between eachstator pole and the motor casing, and that the torque constants aresinusoidal, this permits a smooth torque that is 90.6% of the torque ofa correctly operating motor system. If the motor system is designed tohave a torque margin of some 10.4%, the fall-out of a phase pole groupwould permit full required torque with negligible ripple.

[0045] For high speed loads such as fans or pumps, torque ripple isoften irrelevant. In such cases the currents of the operational fivepole groups can be increased by the square root of {fraction (6/5)} orsome 9%, giving a total power of ⅚×1.09=91.3% of normal power.

[0046] Another purpose of the invention is to provide a brushlesselectric motor system that is capable of delivering a substantial totaltorque or power even for a permanently shortened power switch. If apower switch such as T20 in FIG. 3 is kept permanently shorted asillustrated at 305, any attempt to switch on switch T19 will cause theconventional overcurrent protection circuit in a bridge such as 406 todisable at least switch T19. The bridge logic will also send anovercurrent alarm to the control processor though a protection faultline like 406. In the case where the over current alarm does notidentify the individual switch, the processor could identify the sourceof the problem by selectively disable winding group after group untilthe switch causing the fault is identified. After finding that switchT19 always causes an overcurrent alarm while an activation of switch T20does not, the processor can assume that switch T20 is permanentlyshorted. In the case where the negative rail, shown at 408 in FIG. 4, isat ground level, a short from the right end of coils V1-3 to ground willgive the same results as a short inside the switch T20. A suitableaction is to disable all the four transistors T17-T20. This willeliminate the controlled current in phase coils V1-3.

[0047] However, as the short 305 through transistor T20 always isconducting, an emf from the motor rotation that makes the left side ofcoils V1-3 negative will produce a current loop shown as 306 in FIG. 3.At higher speeds, the fault current shown as 306 can be several timesthe phase current for full nominal power load. For the motor type usedas an example, this current will at higher speed cause a brake torquethat has a peak value of the same magnitude of order as the normal fullpower of the motor. The average brake torque is however only some 10 to20% of the normal full power of the motor as the fault current due tophase lag also gives substantial positive torque for other rotor angles.The brake torque is therefore not large enough to seriously degrade theperformance of the motor system. At high speeds, where the error currentthrough coils V1-V3 will be far higher that their full power current fornormal operation, one of the slow fuses like 401 will break andeliminate the current through coils V1-V3. The current through coilsV1-V3 could otherwise cause their temperature to quickly reach some 500°C., triggering further insulation breakdowns. The graphs of FIG. 8 showthe emf E in coils V1-3, the brake torque T, and the current I for theexample motor for the case where the power switch T20 is short-circuitedand under the assumption that the insulation between turns in the coilshas not broken at the high long term temperature of 512° C. and that nofuses like 401 have blown.

[0048] Another purpose of the invention is to provide a brushlesselectric motor system that has a dramatically reduced risk of phase tophase shortages. This is obtained by the fact that the windings of anyphase like U4-U6 are separated from other phases by separation poles orbalancing poles like 202 and 203.

[0049] Another purpose of the invention is to provide a brushlesselectric motor system that is capable of delivering substantial torqueor power even for a ground to phase short. Such a failure is shown at301 in FIG. 3. Assuming that the DC rails like 408 and 409 are notfloating, this short will cause very large currents through transistorsT11 or T12, thereby causing the conventional overcurrent protectioncircuit, not shown, to disable the bridge and send an alarm to thecontroller 403. In this case, an overcurrent alarm will be obtained assoon as any of the switches T11 and T12 are enabled even when switchesT9 and T10 are disabled. This indicates a short to ground as indicatedat 301. The controller 403 should thereafter disable all the fourtransistors T9-T12 driving the coils U4-U6.

[0050] Yet another purpose of the invention is to provide a brushlesselectric motor system that is capable of delivering substantial outputtorque or output power even for a short circuit inside a phase winding.This is illustrated by the short 302.

[0051] If the motor rotates, the emf caused by the rotor magnetsproduces current/s flowing in the shorted coil/s. This causes a braketorque which is negligible at low speeds but which at higher speeds willcause dramatic heating of the shorted parts. In a motor where phasewindings are located adjacent to each other, this could cause the faultyphase to heat the insulation on neighbouring phases to temperatureswhere its insulating properties are seriously reduced, thus creating onephase short to create temperatures that can cause shorts in otherphases. This can result in that one phase short will create temperaturesthat cause shorts in other phases, thereby causing a total failure ofthe motor system. In the motor of FIG. 2, the intermediate poles such as202 and 203 will dampen the heat transfer from a shorted pole to theadjacent phase windings. Therefore, an internal short in a phase willgive a smaller and uneven torque, but will permit the system to continueto operate with a reduced output torque and/or at a reduced speeds.

[0052] If the short covers all the three coils in a phase winding group,as illustrated in FIG. 3, the result will be an overcurrent trip alarmas soon as both T1 and T4 or T2 and T3 are enabled. At high speeds, theresult will be similar to that described for error type 305 of FIG. 3.The braking torque will have high peak values that to a large extentbalance each other, causing a brake torque that is some 10 times lowerthan the full power torque of the motor. The fuses will break if aninternal phase short circuit affects more than one coil. For example,the short illustrated as 302 in FIG. 3 affects all the three coils W4-W6and would at high speeds cause large currents to flow through the coilsW4-W6 and therefore cause a fuse like 401 to open. In this way, a singlefault insulation breakdown in a phase will in the worst case cause ashort that will create local heating and brake torque from one singlestator pole coil. Compared to the short 302 indicated in FIG. 3, thiswill reduce the brake torque to a third or less. In the diagram of FIG.9 the emf E in coils W4-6, the brake torque T, and the current I areshown for the example motor in a case where the complete coil group W4-6has been shorted. For the example of the figure is assumed that theinsulation between turns in the coils has not broken at the high longterm temperature of 365° C. and that there are no fuses like 401.

[0053] If the short covers only a fraction of the coils W4-W6, thecontroller 403 will note a largely distorted current response from thecurrent sensor 303 and/or a higher temperature detected by thetemperature sensor 402. The normal action is to reduce or eliminate thecurrents through the coils W4-W6 by changing the switch pattern forswitches T1-T4. The temperature of the shorted coil can reach very highlevels, in the example motor some 370° C. at 3000 rpm when a mostdisadvantageous part of one coil is shorted. As the adjacent coils haveno current and only the relatively low and smooth voltage from the emfcaused by the rotation of the rotor, the stress on their insulation islow. This is important as the dielectric strength of high performanceinsulation materials decreases rapidly at high temperatures. The highvoltage transient from the switches appears only in the torque creatingcoils in the other pole groups, which are thermally screened by theintermediate poles like 203.

[0054] Shorts within a winding coil are serious as they can easilycreate local hot spots that are hot enough to destroy the insulation ofadjacent turns in a coil. A highly reliable motor system can be designedso that it can give the required power even if for example a singleinternal coil short according to a worst case occurs. To optimize thedesign, it is important to distinguish between the temperatures that aninsulation system can withstand during 20000 hours during normalconditions and the far higher temperature that it can withstand in atime period during which the system must work having a serious faultthat generates heat. In an aircraft application this time may be 20hours, i.e. the flight time from error identification alarm to landing.A design principle for an electric motor system based on thisrequirement could comprise the following steps:

[0055] determining the expected maximum time during which the systemmust be able to operate after identification of a fault condition,

[0056] determining the maximum or limit temperatures that are permittedin different components during different operating conditions andpossible also that give a negligible risk of additional failuresoccurring during the maximum time,

[0057] calculating the maximum temperature caused in different parts ofthe motor system during each fault condition assuming that the motorsystem is to deliver necessary power, and

[0058] dimensioning the system so that the required power can bemaintained during the maximum time without surpassing the maximumtemperatures in the case where a fault would occur.

[0059] In the case of internal coil shorts, an object of the inventionis to ensure that the hot spot temperature is kept within limits thatpermits the insulation system of adjacent winding layers or turns tostay intact. FIGS. 11 and 12 show a design compromise to achieve thisfor a motor having internal ventilation. FIG. 1 shows a design where thecopper filling of the stator slot is kept as high as possible using thewire diameter required. By removing the two wire turns shown at 1101,the performance of the motor system may suffer. On the other hand, thearea of the ventilation channel is increased, permitting a bettercooling of the coils and thereby permitting a higher rotor speed in thecase where a coil obtains an internal winding short.

[0060]FIG. 12 also shows high permanent magnets. By keeping the magnetshigher than necessary for normal operation, the ventilation area 1202between magnets is increased and the internal flux weakening of themagnets due to stator currents decreases. This gives higher costs ofmanufacture but permits higher rotor speeds in the case of a shortedstator coil. A shorted stator coil will at high speeds give localheating with high power. Presently available magnet materials will givea lower flux at high temperatures and will more easily be permanentlydemagnetised at high stator currents at high temperatures. Cooling anddemagnetisation margins of the rotor magnets are therefore importantdesign criteria for high reliable motor systems.

[0061] Depending on the conditions given by the motor application, thespeed of the motor can be reduced when a single coil short is detectedso that the coil temperature can be kept at a level that reduces therisk that the heat from one internal coil short will cause secondaryshorts within the same coil during the necessary operation life timefrom a detected fault to a service intervention. For the example motor,this is at some 2200 rpm with a hot spot temperature of some 290° C. andan average brake loss of some 10% of normal operation torque. In otherapplications, the system can be designed so that the normal operatingspeed gives a temperature rise in a shorted coil that is acceptable forthe required operation life time from a detected fault to a serviceintervention.

[0062] The circuit diagram of FIG. 5 illustrates a design of theswitching circuitry and the phase windings having additional switchescomprising two groups of triple switches 501-503 and 504-506. At highspeeds, all switches 501-506 are non-conducting or open and the systemoperates as the system of FIG. 3. At low speeds, one of the two tripleswitch groups can be activated, i.e. all the switches in the group areclosed or made to conduct. If the switches 504-506 are activated, thecoils W4-6 and W1-3 are effectively connected in series as long astransistors T7-T18 are disabled. This permits the use of full currentsin all phases but reduces the power switch losses to half, as 12 of the24 power transistors are permanently disabled/open. It will also reducethe iron losses due to the current control switching as each pole groupwinding will face only half of the full rail voltage. It thereforepermits substantial loss reduction, specially at low speeds. To obtainthis advantage, only one set of triple switches such as 501-503 isnecessary, and the switches 504-506 can then be deleted. It also permitsthe motor system to operate at full torque up to medium speeds even if acomplete hex bridge, such as T7-T12, fails completely, for example ifits internal control power system fails. Should hex bridge T1-T6 fail,switches 501-503 of the other group should be activated.

[0063] As is obvious for those skilled in the art, the design shown anddescribed above can be varied in different ways.

[0064] The shown implementation uses six galvanically separated phasewinding groups each driven by four switches as 4 hex switch modules.With reduced performance in fault operation, three coil groups with atotal of 12 switches can be used. Another alternative is to have the sixpole groups shown connected as two sets of normally Y-connected windingsets each driven by six switches as shown in FIG. 1. Obviously, thenumber of poles in each pole group and the number of pole groups can bevaried.

[0065] The shown implementation arranges the thermal shielding betweenstator coils in the same pole group by unwound poles like the balancingpoles disclosed in U.S. Pat. No. 5,442,250. An alternative way to obtainthermal shielding between coils belonging to different phases is shownin FIG. 10. In this case a prior art stator such as the one shown inU.S. Pat. No. 5,929,549 is modified so that every second stator polewinding is eliminated and the remaining coils have got almost twice thewinding copper area. This solution creates a thermal shielding but willget larger winding ends, a higher thermal resistance from the outerlayers to the stator iron pole and a higher risk of local shorts beingspread as the possible shorted winding turns are buried deeper inside awinding.

[0066] The shown implementation has an internal rotor and externalstator. Obviously an inverted arrangement as shown in the cited U.S.Pat. No. 5,442,250 can be used.

[0067] The shown implementation has a rotating rotor and correspondingstator. Obviously the principles shown can be applied also to linearmotors comprising a linear stator and a member, a linear slide, that haspermanent magnet poles and is movable in relation to the stator.

[0068] The shown implementation has slow fuses. Other high currentlimiting components such as PTC:s (Positive Temperature Coefficientdevices) having very steep switch-over characteristics can be used inplace of fuses.

[0069] The shown implementation has all the coils like W1-3 and allfuses for the groups connected in series with each other. Many otheralternatives are possible, the other extreme being having each coilconnected individually to the switch network through a fuse.

[0070] While specific embodiments of the invention have been illustratedand described herein, it is realized that numerous additionaladvantages, modifications and changes will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, representative devices andillustrated examples shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents. It is therefore to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin a true spirit and scope of the invention.

1. A brushless electric motor system comprising: a rotor or linear slidehaving north and south poles, a stator comprising poles and electricalphase windings, each phase winding including coils wound around thepoles, a switching network comprising power switches intended to beconnected to an electric power source and also connected to thewindings, and a control device controlling the switching network tosupply the phase windings with electric power at appropriate times inorder to obtain a torque or force on and thereby a movement of the rotoror linear slide, characterized in that the coils of each phase windinghave no metallic conductive connection with coils in the other phases,the switching network comprising for each phase at least one group offour power switches arranged in an H-configuration and connected betweena positive conductor or rail intended to be connected to a positiveterminal of the power source and a negative conductor or rail intendedto be connected to a negative terminal of the power source.
 2. Abrushless electric motor system according to claim 1, characterized inthat the coils of each phase winding are divided into winding groups,each winding group having no metallic conductive connection with otherwinding groups, the switching network comprising for each winding groupfour power switches arranged in an H-configuration between the positiveconductor or rail and the negative conductor or rail.
 3. A brushlesselectric motor system comprising: a rotor or linear slide having northand south poles, a stator comprising poles and at least two electricalwindings, each winding including coils, each coil arranged around asingle stator pole, each winding having no metallic connection with theother winding/s, a switching network comprising power switches intendedto be connected to an electric power source and also connected to thecoils of the windings, and a control device controlling the switchingnetwork to supply the windings with electric power at appropriate timesin order to obtain a torque or force on and thereby a movement of therotor or linear slide, characterized in that each coil of each one ofthe windings is separated from the coils in the other winding/s byunwound stator poles.
 4. A brushless electric motor system comprising: arotor or linear slide having north and south poles, a stator or linearslide comprising poles and electrical phase windings, each phase windingincluding coils, a switching network comprising power switches intendedto be connected to an electric power source and also connected to thephase windings, and a control device controlling the switching networkto supply the phase windings with electric power at appropriate times inorder to obtain a torque or force on and thereby a movement of the rotoror linear slide, characterized in that each coil of each one of thephase windings is connected to other coils in the same phase windingand/or to the switching network through high current limiting devices,in particular fuses.
 5. A brushless electric motor system accordingclaim 4, characterized in that the coils of each phase winding aredivided in at least two groups, the coils in one group having nometallic conductive connection with the coils in other groups.
 6. Abrushless electric motor system according to any of claims 1-5,characterized by a current sensor connected to the control device andsensing the current in each phase winding or in each winding group.
 7. Abrushless motor system according to any of claims 1-5, characterized inthat the switching network includes devices for sensing switchedcurrents, that the switching network and/or the control device is/arearranged to detect whether the magnitudes of the sensed switchedcurrents exceed a safety level, and that the control device is arrangedto identify at least one fault condition including shorts in phasewindings, shorts from phase windings to ground and failures of the powerswitches, disable those of the power switches that drive phase windingsor winding groups for which, in the coils or the power switchesassociated therewith, a fault condition has been identified, and usestill operating parts of the brushless motor system to ensure that thebrushless motor system can operate to move the rotor or linear slide inspite of fault conditions albeit with reduced peak performance.
 8. Abrushless motor system according to any of claims 1-5, characterized inthat the poles of the rotor or linear slide are derived from permanentmagnets.
 9. A brushless electric motor system according to any of claims1-5, characterized by temperature sensors located close to at least onecoil in each coil group, at least one individual temperature sensorhaving a relatively high resolution arranged for each coil group, thetemperature sensors arranged to make measurements of the temperature ofsaid at least one coil in each coil group.
 10. A brushless electricsystem according to claim 9, characterized in that the control device isarranged to perform according to a control policy to optimise theperformance of brushless electric motor system for fault conditionsidentified in system components using information from the temperaturesensors to adjust, in particular increase the intensity of, stillcontrollable currents flowing through phase windings and/or coil groupsso that a maximum output power or maximum performance in regard of thetorque or force is obtained without surpassing predetermined temperaturelimits.
 11. A brushless electric motor system according to claim 10,characterized in that the predetermined temperature limits used arebased on a relatively negligible risk for additional faults or failuresof system components during an expected maximum time during which thebrushless electric motor system must be able to operate afteridentifying one of the fault conditions.
 12. A brushless electric motorsystem according to any of claims 1-5, characterized by additionalswitches connected between coil groups of each phase winding andcontrolled by the control device, one individual additional switcharranged for each phase winding and connected between conductor ends oftwo coil groups of the phase winding, the control device arranged tocontrol the additional switches to be open in the case where no faultconditions has been identified and to control one of the additionalswitches to be closed in the case where a fault condition in anassociated group of power switches have been identified, the associatedgroup of power switches including those of the power switches that areconnected to said conductor ends.
 13. A brushless electric motor systemaccording to claim 12, characterized in that the additional switches areconnected between all the coil groups of each phase winding, oneindividual additional switch arranged for each coil group.
 14. Abrushless electric motor system according to any of claims 1-5,characterized by additional switches connected between coil groups ofeach phase winding and controlled by the control device, one individualadditional switch arranged for each phase winding and connected betweenconductor ends of two coil groups of the phase winding, the controldevice arranged to control the additional switches to be open for highrotation speeds of the rotor or high movement speeds of the linear slideof the brushless electric motor system and to control all the additionalswitches to be closed for low rotation or movement speeds, the powerswitches of an associated group of power switches not being used forswitching at the low rotation or movement speeds, the associated groupof power switches including those of the power switches that areconnected to said conductor ends.
 15. A method of designing an electricmotor system, characterized by the steps of: determining an expectedmaximum time during which the electric motor system must be able tooperate after the identification of a fault condition, determiningmaximum temperatures permitted in different components of the electricmotor system for different operating conditions and giving a negligiblerisk of additional fault conditions occurring in the electric motorsystem during the expected maximum time, calculating maximumtemperatures caused in the different components of the electric motorsystem for each of the fault conditions assuming that the motor systemis to deliver a desired power, and dimensioning the electric motorsystem so that the desired power can be delivered during the maximumtime without exceeding the maximum temperatures in the case where one ofthe fault conditions would occur.
 16. A method of designing an electricmotor system, characterized by the steps of: determining an expectedmaximum time during which the electric motor system must be able tooperate after the identification of each one of fault conditionspossible to occur in the electric motor system and to still deliver adesired output power, determining maximum or limit temperaturespermitted in different components of the electric motor system fordifferent operating conditions of the electric motor system, calculatingmaximum temperatures caused in different parts of the electric motorsystem for each of the fault conditions, and dimensioning the electricmotor system so that the desired power can be delivered during theexpected maximum time without exceeding the determined maximum or limittemperatures in the case where one of the fault conditions would occur,wherein, in determining the maximum or limit temperatures, the maximumor limit temperatures are determined by the requirement that thereshould be a relatively negligible risk for additional faults occurringduring the expected maximum time during which the electric motor systemmust be able to operate after an identification of one of the faultconditions.